Suppressed Phenotypes during Oral Carcinogenesis Oral squamous cell carcinoma is a condition that will kill approximately half of patients afflicted within five years of diagnosis and may leave surviving patients with severe esthetic and/or functional compromise. Oral cancer progresses through a series of morphologic changes, which if treated early, vastly improve prognosis. Though cancer is widely believed to be the result of chromosomal alterations leading to the activation of oncogenes and suppression of antioncogenes, the actual biochemical and molecular changes preceding the morphologic progression of oral lesions are poorly understood. The principle aim of this work is the isolation of normal-specific genetic sequences that may be involved in the suppression of malignant phenotypes during oral carcinogenesis. To test that chromosomal rearrangements and deletions during oral cancer development result in the loss of genetic suppressors of malignant transformation, we used the well-established hamster cheek pouch carcimona (HCPC) model. Normal and malignant oral keratinocytes from the cheek pouches of the Syrian hamster were cultured. Extensive phenotypic assays were preformed to confirm the normal and malignant behaviors. Complementary DNA (cDNA) libraries have been constructed from the normal and malignant hamster oral keratinocytes representing expressed genes in each population. Genetic sequences that are uniquely and/or preferentially expressed in the normal hamster oral keratinocytes were isolated by the technique of subtractive hybridization. A subtraction library of the genetic clones isolated has been constructed. From this library, 130 candidate clones were identified as Type I (normal)- specific by differential colony hybridization. Southern blot analysis of these 130 subtraction clones demonstrated that 29 clones were Type I-specific. The normal-specific expression of 4 of 7 substraction clones were confirmed by Northern blot analysis. Two of these clones demonstrated the loss of a genomic fragment upon Southern blot analysis. This finding is suggestive of a loss of heterozygosity, a common observation associated with several important tumor suppressor genes. Currently, the full-length cDNAs represented by these substraction clones are being isolated by a 5' and 3' RACE (rapid amlification of cDNA ends) approach. The availability of these regents will allow us to identify and re- express these putative TSGs in maligant oral keratinocytes to systematically examine the functional significance of these genes in the suppression of various transformation phenotypes. This approach to the identification of tumor suppressor genes should hasten the identification as well as understanding of tumor suppressors which are potentially critical to the biology, diagnosis and treatment of oral cancer. Key words: tumor suppressor genes, oral carcinogenesis, subtractive hybridization